A vacuum heat treatment apparatus, which heat-treats a source material put in a crucible to form a desirable material, performs the heat treatment in a vacuum state to prevent surrounding contamination. According to the vacuum heat treatment apparatus, an adiabatic member is placed in a chamber maintained in the vacuum state, and a heater is placed in the adiabatic member, thereby heating the source material.
However, the material (reaction product), which is produced in the middle of the reaction due to the reaction between the crucible and the source material, may stick to the inner wall of the crucible. Since the reaction product is different from the material of the crucible, the thermal stress is applied to the crucible due to the thermal expansion coefficient difference between the above heterogeneous materials. In the worst case, the crucible may be broken due to the thermal stress in the middle of the reaction. Therefore, the cost for the replacement of the crucible is excessively increased, so that the productivity may be lowered.
According to the related art, a buffer part is provided in the reaction container so that the reaction product is prevented from being deposited on the inner part of the crucible. Accordingly, the crucible can be prevented from being cracked and damaged due to the thermal expansion difference between the deposited reactant or the deposited product and the crucible.
In addition, according to the related art, the shape of the reaction container is modified, thereby preventing the crucible from being broken and cracked due to the deposition of the reaction product into the crucible. In other words, the thermal expansion difference between the crucible and the reaction product can be compensated by modifying the shape of the reaction container.
However, even though the above scheme is employed, if the reaction product is excessively deposited into the crucible, the crucible may be cracked and broken. In other words, since the thermal expansion coefficient between the heterogeneous materials of the reaction product and the crucible is significantly made, the above schemes have the limitation in preventing the stress applied to the crucible due to the thermal expansion coefficient between the heterogeneous materials.
Therefore, there are required a scheme capable of preventing the crucible from being cracked or broken due to the stress resulting from the thermal expansion difference between the reaction product and the crucible without the modification of the shape or the structure of the crucible.